In a normal heart, cells of the sinoatrial node (SA node) spontaneously depolarize and thereby initiate an action potential. This action potential propagates rapidly through the atria (which contract), slowly through the atrioventricular node (AV node), the atriventricular bundle (AV bundle or His bundle) and then to the ventricles, which causes ventricular contraction. Thus, in a normal heart, ventricular rhythm relies on conduction of action potentials through the AV node and AV bundle.
Regarding the AV node, it is a small subendocardial structure within the interatrial septum, anterior and superior to the coronary sinus. The AV node has extensive autonomic innervation and an abundant blood supply from the large AV nodal artery, which is a branch of the right coronary artery in approximately 90 percent of the population, and from septal branches of the left anterior descending coronary artery (circumflex artery). The AV node forms part of the only “normal” electrical connection between atria and ventricles. While various conduction subpathways may exist in the AV node or AV nodal region, the AV node is known to transmit impulses slowly via at least one pathway, e.g., requiring approximately 60 ms to approximately 130 ms to traverse about 1 cm of node tissue. In general, slowing of an impulse by AV nodal tissue protects the ventricles by typically not allowing all impulses above a certain rate to pass through to the ventricles, which, in turn, prevents the ventricles from racing in response to a rapid atrial rhythm. Under some circumstances, the AV node blocks all impulses to the ventricles. Further, clinical ablation of the AV node can also block all impulses to the ventricles.
En route to the ventricles, action potentials pass via Purkinje fibers, which typically emerge from the distal region of the AV node and converge gradually to form at least part of the AV bundle. Blood supplies the AV bundle from the AV nodal artery and septal branches of the left anterior descending artery. The AV bundle has relatively sparse autonomic innervation and is somewhat encased within a collagenous skeleton. Destruction of the AV bundle, for example, through ablation, may also block all impulses to the ventricles.
While ablation of a patient's AV node and/or AV bundle has been shown to block unwanted conduction of action potentials to the ventricles, these procedures are irreversible; thus, a need exists to slow and/or block action potentials in a reversible manner. As described herein, various exemplary methods and/or exemplary devices stimulate nerves to effectively slow and/or block conduction of action potentials through a patient's AV node and/or AV bundle. Further, various exemplary methods and/or devices accomplish such tasks in a reversible manner.